> Maybe a video would help diagnose it further? I'm very interested in what this could> be.

The only other thing I can think of is ~possibly~ a blanking/sync circuit issue. If the black spot is oriented in the top left of monitor, a weak component might be keeping the beam blanked a little too long. Maybe a slow cap hasn't had enough time to charge after the long retrace from bottom to top? This would allow the potential to behave differently at a lower res when the game starts...

Tough to say without a schematic to reference, or even a model number. Even tougher to buy that explanation since he said it was new.